The present invention relates to a small sized printer for use in recording information read out of a small data processing apparatus called a "micro-computor". More particularly, the invention relates to a so-called "flying printer" in which characters arranged on the peripheral face of a continuously rotating print drum are selectively struck by a printing hammer.
A flying printer of this type, in general, comprises a printing mechanism for driving a hammer for selectively striking type face arranged on the peripheral face of a continuously rotating print drum, a paper transfer mechanism for intermittently transferring a paper to be printed between said print drum and a group of hammer means, and a mechanism for imprinting characters on the paper when the type faces on the print drum are struck by said hammer means.
There is known a flying printer in which the printing mechanism utilizes electromagnetic means for printing selected characters on a continuously rotating print drum, a hammer being directly driven by said electromagnetic means to strike the selected printing characters.
There is also known a small sized flying printer in which a hammer rather than being directly driven by electromagnetic means, is driven by mechanical means including a continuously rotating ratchet wheel and a trigger lever which moves when struck by a tooth of said ratchet wheel, whereby the electromagnet may be reduced in the size and made compact.
In such a small sized flying printer, in general, the hammer struck by the trigger lever is moved more rapidly than the trigger lever and it strikes printing characters on the print drum; however, while the hammer is returning to the stand-by position, it may be hit again by the trigger lever before the trigger level returns to a rest position so that double printing may therefore be caused. In order to avoid such double printing, it has been tried to stop the movement of the trigger lever after the trigger lever strikes the hammer. In such a small sized flying printer, however, the force with which the hammer strikes the type face is weak, and there is brought about a disadvantage that a clear copy cannot be obtained in a single printing operation.
It also is known for the trigger lever to be connected to the hammer for avoiding the above-mentioned double printing. However, in a flying printer including such means, the trigger lever is turned when it receives the hammering action from the ratchet of the ratchet wheel, and the hammered portion slips and the trigger lever is removed from the ratchet before the trigger lever receives sufficient energy from the rotating ratchet to effect satisfactory printing. Accordingly, the striking force of the hammer is weakened, and it sometimes happens that when the hammer returns to the stand-by position, it strikes the printing characters again by the force of the reaction thereby causing double printing.
As the paper transfer mechanism of a flying printer, there is known a mechanism in which a pair of tractors comprising a belt having pins to be engaged with perforations in a form printing paper are disposed above and below the printing position and by intermittently driving said tractors, the paper to be printed is intermittently transferred between a continuously rotating print drum and a group of hammers disposed in parallel to each other and opposing the print drum. In this known paper transfer mechanism when the print drum is struck by the hammer, the paper is pulled and moved in the direction of rotation of the print drum during the time while the drum is in contact with the hammer, and if another hammer strikes the print drum during this time, the line of the printed letters is readily disordered and becomes irregular. Accordingly, in order to eliminate the defect, it is necessary to provide a mechanism for pressing the paper only at the moment the hammer strikes the print drum and to control the timing of this paper-pressing operation. Recently the need for a small sized flying printer cooperative with a small sized information-processing apparatus called " mini-computor" or "micro-computor" has become urgent. When the above-mentioned conventional paper transfer mechanism is adopted, it is very difficult to provide a small flying printer meeting such a requirement.
As the mechanism for providing the ink or coloring material to be used in printing on paper, there is known a mechanism in which an ink ribbon wound on a pair of spools is positioned between the print drum and a group of hammers, one spool is intermittently turned at every printing operation to transfer the ribbon and the direction of the transfer of the ribbon is periodically reversed. Further, it is known that when the diameter of the coil of ink ribbon remaining on the spool is reduced to a certain value, this value is detected and the reversal of the transfer direction is effected. In this known mechanism, since the above reversal is effected while some ribbon still remains on the spool, a certain portion of the ribbon is not used, resulting in an economic disadvantage. As the mechanism for moderating this disadvantage, there is known a mechanism in which eyelet disposed on both the ends of the ink ribbon are detected to effect the above reversal. In such a mechanism, the reversing operation is often uncertain, and when the ink ribbon is guided between the print drum and a group of hammers, the ink ribbon may be bent at an acute angle and a considerable tension may be applied to the ribbon, as a result of which the above reversing operation becomes difficult.